The present invention relates to plastics material pipe tees. The invention is particularly suited for application to pipe tees for joining pipes of 80 mm diameter or more, and particularly for use in pressure pipe installations.
Plastics material pipes and fittings have widespread application, but are not widely used for applications in which they are subjected to relatively high pressures. A major reason for this is the short life of plastics pipe tees when subjected to cyclic variations in pressure such as those which occur in a pipeline when a valve is opened or closed or which occur in the region downstream of a pump. In these situations, plastic pipe tees have a tendency to develop cracks at the crutch points which lie at the junction of the branch and the straight body of the tee in the centre plane which contains the axes of both the branch and the body. This problem is peculiar to plastics tees and does not affect cast iron tees, and thus cast iron tees are often preferred for these situations.
Attempts have been made to overcome this defect by stiffening the tee at the crutch points by thickening at those points or by the provision of ramps moulded with the tee to extend between the body and the branch in the region of the crutch. However, these modifications have been found to give no significant increase to the life of the tee. Furthermore, thickening in the region of the crutch promotes unequal heat transfer during the cooling process after moulding, resulting in cooling stresses which weaken the tee.